Known examples of devices configured to measure a deep body temperature include a device called non-heating simple deep body thermometer (hereinafter referred to as non-heating deep body thermometer) or the like is known.
Known examples of the non-heating deep body thermometer include a thermometer configured to measure a deep body temperature by using a single heat flux sensor obtained by mounting temperature sensors on top and bottom surfaces of a heat insulator having a relatively wide area and a thermometer configured to measure a deep body temperature by using two such heat flux sensors.
Referring to FIG. 1, the former non-heating deep body thermometer is first described.
For measurement of a deep body temperature with a non-heating deep body thermometer using a single heat flux sensor, the heat flux sensor is brought into intimate contact with the body surface as illustrated in FIG. 1(A).
As illustrated in FIG. 1(B), a temperature Tt of a heat insulator bottom surface measured by the heat flux sensor (temperature sensor closer to bottom surface of heat insulator) in intimate contact with the body surface is lower than a deep body temperature Tb. A temperature Ta of a heat insulator top surface measured by the heat flux sensor (temperature sensor closer to top surface of heat insulator) in intimate contact with the body surface is lower than the temperature Tt. A thermal equivalent circuit of the structure illustrated in FIG. 1(A) is expressed by FIG. 1(C). Rx and R1 are a thermal resistance value of subcutaneous tissue, which is a non-heating body, and a thermal resistance value of the heat insulator, respectively.
When the temperature of each part of the heat flux sensor in intimate contact with the body surface is stabilized, the quantity of heat passing through the non-heating body in a unit time and the quantity of heat passing through the heat insulator in a unit time becomes equal to each other. Specifically, when the temperature of each part of the heat flux sensor is stabilized, Equation (1) is established.(Tb−Tt)/Rx=(Tt−Ta)/R1  (1)
Thus, when the temperature of each part of the heat flux sensor is stabilized, the deep body temperature Tb can be calculated by Equation (2) obtained by solving Equation (1) for Tb.Tb=Tt+(Tt−Ta)·Rx/R1  (2)
The non-heating deep body thermometer of the type using a single heat flux sensor is configured to calculate the deep body temperature Tb by Equation (2). The Rx value, however, differs depending on location and differs among individuals. The deep body temperature Tb calculated by Equation (2) using a fixed value as the Rx value thus includes a measurement error corresponding to a difference between the Rx value used and the actual Rx value.
A non-heating deep body thermometer developed such that the measurement error is prevented from being included in the measurement result of the deep body temperature Tb is a non-heating deep body thermometer of a type configured to measure a deep body temperature by using two heat flux sensors (see, for example, PTL 1).
For measurement of a deep body temperature with this type of non-heating deep body thermometer, two heat flux sensors are brought into intimate contact with the body surface as illustrated in FIG. 2(A).
As illustrated in FIGS. 2(A) and 2(B), a temperature of a heat insulator top surface and a temperature of a heat insulator bottom surface measured by a thermal resistance value of a heat insulator in one heat flux sensor (hereinafter referred to as first heat flux sensor) are referred to as Ta and Tt, respectively, and a temperature of the heat insulator top surface and a temperature of the heat insulator bottom surface measured by the other heat flux sensor (hereinafter referred to as second heat flux sensor) are referred to as Ta′ and Tt′, respectively. In this case, a thermal equivalent circuit of the structure illustrated in FIG. 2(A) is expressed by FIG. 2(C). Rx, R1, and R2 are a thermal resistance value of subcutaneous tissue, which is a non-heating body, a thermal resistance value of the heat insulator in the first heat flux sensor, and a thermal resistance value of the heat insulator in the second heat flux sensor, respectively.
Thus, Equation (2) is established for the first heat flux sensor, and Equation (3) is established for the second heat flux sensor.Tb=Tt′+(Tt′−Ta′)·RX/R2  (3)
Eliminating Rx from Equations (2) and (3) enables Equation (4) to be obtained.
                    [                  Math          .                                          ⁢          1                ]                                                            Tb        =                                            R              ⁢                                                          ⁢                              2                ·                Δ                            ⁢                                                          ⁢                              T                ·                                  Tt                  ′                                                      -                          R              ⁢                                                          ⁢                              1                ·                Δ                            ⁢                                                          ⁢                                                T                  ′                                ·                Tt                                                                        R              ⁢                                                          ⁢                              2                ·                Δ                            ⁢                                                          ⁢              T                        -                          R              ⁢                                                          ⁢                              1                ·                Δ                            ⁢                                                          ⁢                              T                ′                                                                        (        4        )            
The use of a ratio k (=R2/R1) of R2 to R1 enables Equation (4) to be transformed to Equation (5).
                    [                  Math          .                                          ⁢          2                ]                                                            Tb        =                                                            k                ·                Δ                            ⁢                                                          ⁢                              T                ·                                  Tt                  ′                                                      -                          Δ              ⁢                                                          ⁢                                                T                  ′                                ·                Tt                                                                                        k                ·                Δ                            ⁢                                                          ⁢              T                        -                          Δ              ⁢                                                          ⁢                              T                ′                                                                        (        5        )            
The non-heating deep body thermometer of the type using two heat flux sensors is configured to calculate the deep body temperature Tb by Equation (4) or Equation (5).